1. Field of the Invention
The field of invention relates generally to optical measurement tools and, more particularly, to a method for correcting systematic error in such tools.
2. Description of the Related Art
Characteristics of semiconductor devices are directly dependent on the shapes and dimensions of one or more layers and/or features of a device. During a fabrication process, a drift in one or more process parameters can result in deviations on the device critical dimensions (CDs), potentially rendering the devices useless. Optical metrology has been traditionally employed to monitor semiconductor fabrication processes. Scanning Electron Microscopy (SEM) has been in the forefront in this area. Commonly referred to as CD-SEM (Critical Dimension Scanning Electron Microscopy), this form of optical metrology poses two major disadvantages. One is that the measurement process is destructive and the other is that it cannot be used in-situ, which prohibits its deployment in integrated metrology.
Optical digital profilometry (ODP) has emerged recently as a metrology system that overcomes the above mentioned short-comings of CD-SEM. The basis of ODP is that quantities resulting from a measurement of a layer, a series of stacked layers, or a device structure with a specific profile characterized by specific material properties, layer thicknesses, and/or CDs are unique to that profile. A library of profiles may be created to represent a set of layers and/or device structures, each with its own unique profile.
A library of films and/or profiles with corresponding predicted spectra can be generated using scatterometry. Scatterometry is an optical measurement technology based on an analysis of one or more wavelengths of light scattered from a layer or array of layers and/or device structures. The device structures may be a series of photoresist gratings or arrays of contact holes on a test sample. Scatterometry is a model-based metrology that determines measurement results by comparing measured light scatter against a model of theoretical spectra. A profile of the given test sample is extracted by searching the library for a match of the measured spectra with theoretical spectra in the library and once a match is found, the corresponding profile is taken to be the profile of the given sample.
The methodology of ODP is computer-intensive since it involves generation of one or more libraries of predicted spectra as well as searching of a master library, or one or more derivatives of the master library, for matching spectra. The size of the library generally governs the resolution of the final result. Generation of one or more libraries typically involves repeating a prediction process for a series of layers and/or profile shapes to create a series of corresponding scatterometry spectra. Prediction of scatterometry spectra is provided by a numerical solution of governing Maxwell's equations. Each layer, stacked layer, and/or profile is translated into a theoretical model that factors in physical parameters such as optical properties of the semiconductor materials. Maxwell's equations are applied with appropriate boundary conditions to form a system of equations that are numerically solved for example using Rigorous Coupled Wave Analysis (RGWA).
Measurements taken of the same sample on multiple scatterometry tools or optical metrology systems yield different scatterometry spectra, since each tool is prone to tool specific systematic error sources. Each master library, and/or one or more derivatives of each master library, is hardware dependent because measured spectra for each profile are affected by the tool specific systematic error sources. As a result, the master library and/or derivatives of each master library needs to be regenerated for each metrology tool. Library regeneration is a time consuming process that may impact availability of a scatterometry tool in a manufacturing environment. In many cases, the scatterometry tool is used as in-situ process control for one or more semiconductor manufacturing processes, such as lithography or dry-etch. The manufacture of semiconductors is an extremely expensive process and a reduction in scatterometry tool availability is expensive, if not intolerable.